Polyphosphazene polyelectrolytes are potent immunostimulants. Their in vivo performance has been demonstrated for various antigens in a number of animal models. To improve understanding of the mechanism of action, we performed a comparative study in a model system: bovine serum albumin, BSA-poly[di(carboxylatophenoxy)phosphazene], PCPP, in vitro and in vivo. Multi-angle laser light scattering (MALLS) and size-exclusion HPLC methods were used to investigate polyphosphazene-protein formulations in an attempt to establish correlations between their physicochemical behavior and immunostimulating activity. These studies revealed the formation of water-soluble noncovalent protein-polymer complexes in the system. It was shown that both the amount of bound protein and the complex conformation could play an important role in the in vivo performance of the polyphosphazene polyelectrolytes.